1. Field of the Invention
The present invention relates to an image pickup apparatus and image pickup method for picking up an object using a solid-state image pickup element.
2. Related Background Art
There is an image pickup apparatus such as a digital camera for electronically or magnetically recording and reproducing on various mediums still images or moving images picked up by a solid-state image pickup element such as a CCD or MOS sensor.
In order to reduce price or improve performance of such an image pickup apparatus, there is an image pickup element (refer to FIG. 8) as described in U.S. Pat. No. 5,965,875 where it is noted that a light absorption coefficient of a semiconductor substrate used in an image pickup element is different according to a light wavelength.
In this patent, there are described a principle of an image pickup element having a three-layer photodiode structure where photodiodes are formed with a triple-well structure, and a pixel circuit. According to the patent, the photodiodes are sequentially diffused from a surface of a p-type silicon substrate, and an n-type layer, a p-type layer, and an n-type layer are deeply formed in this order, so that pn-junction diodes are formed over three layers in the depth direction of the silicon in the same pixel. As a light incident into the diode from the surface has a longer wavelength, it penetrates more deeply. Since the incident light wavelength and the light absorption coefficient indicate a value specific to the silicon, a current is separately detected from the three-layer diodes, so that an optical signal having a different wavelength band can be detected.
The depth of the pn-junction of the three-layer diode is designed so as to cover the wavelength band of a visible light. Three signals are calculated so that signals where an object image is color-separated into three colors of R color, G color, and B color can be obtained.
By the use of such an image pickup element, it is possible to eliminate a color filter for performing color separation of the object image or an optical low-pass filter for preventing moiré due to spatial sampling. Further, in the respective colors obtained by color-separating the object image, centers of gravity of sensitivity match with each other, so that color moiré is advantageously difficult to be produced.
However, in the image pickup element for performing color separation in the depth direction of a semiconductor, when a noise is present, whose occurrence position is unstable in the depth direction of the semiconductor like a dark current, a different noise occurs for each color even in the same pixel, thereby deteriorating the image.
Further, in the image pickup element as described above, among the three photodiodes corresponding to the respective colors in the same pixel, two photodiodes adjacent in the vertical direction are capacitive-coupled to each other via the pn-junction. Further, as charges generated in photoelectric conversion are accumulated in the photodiode, the capacity of the photodiode changes. Therefore, a potential of a photodiode in one layer is influenced even by the amount of charges accumulated in other photodiodes. Further, when the photodiode in the uppermost layer is saturated, the excess charges in this diode are flowed into the n-type area of the photodiode in the lowermost layer beyond a potential barrier made of a second p-type layer from the top. Therefore, when there is a white defect or the like due to a local crystal defect of a semiconductor, even a color signal at the depth where there is no defect is influenced, thereby deteriorating the image.